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Need your experience please - bowl cracked after 6 months (Read 151 times)
 
Alex Hixenbaugh
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Need your experience please - bowl cracked after 6 months
Nov 29th, 2018 at 7:30pm
 
Hello - I made a layered bowl consisting of two 2 inch pieces of Bishops wood (Bischofia Javanica) separated by a 1/4" layer of Padauk. It is a yarn bowl which has a slot cut in it to hold the yarn as it is pulled from the bowl.  The bowl is approx. 1/8" thick and after about 6 months of use has cracked through the Padauk - not along the glue line but actually through the Padauk (picture attached).  Does anyone have any idea of why it would fail in this way and what could be done to prevent it?
Thank you very much for your help.
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Ed Weber
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Re: Need your experience please - bowl cracked after 6 months
Reply #1 - Nov 30th, 2018 at 9:51am
 
At first glance, the Padauk failed but it was pushed to failure by the Bishop wood.
Difficult to tell without seeing the entire piece but the grain alignment between the layers might not have been optimum, also the grain direction in the Paduak may have played a part as well.
The main issue was most likely the Moisture content of the Bishop wood.
Even if the wood is dry enough for "standard" turning, there are still internal forces or stress present. Typically most any bowl will move and become slightly out of round over time. In you case, cutting the yarn slot relieved tension on one side and gave the wood room to move, so it did. The Padauk could not move in the same manner as the Bishop wood and failed.
Another thing that most likely contributed is the wall thickness. While 1/8" wail thickness might usually be fine, cutting the yarn slot simply weakened the integrity of the wall enough and as I said above the internal stress in the wood won out.
The relative humidity where it was made and where it's used can also play a part in the amount of wood movement.

Things to consider/remember,
Wood species
Moisture content,
Grain alignment,
Grain direction,
Wall thickness,
You can rough turn and let the wood dry, move and stabilize over a period of time, this method definitely helps but is not a 100% guarantee.
You could leave the wall a bit thicker in vessels where you plan to cut a slot (relieve tension)
I'm sure other can add more
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Alex Hixenbaugh
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Re: Need your experience please - bowl cracked after 6 months
Reply #2 - Nov 30th, 2018 at 1:24pm
 
Thanks for the response Ed.
I usually try to orient the grain in the same direction for all the pieces but in looking at this bowl, the top piece and the padauk are off almost 90 degrees from the bottom piece (the top and padauk end grain is aligned with the side grain of the bottom piece).  I've seen movement in other yarn bowls I've done but none have cracked yet (fingers crossed) so maybe it's the grain orientation that is the culprit here.
Your suggestion of twice-turning these bowls also makes sense as does a little more wall thickness.
I've been experimenting with using segmented feature rings in between these solid blocks of wood - do you think that would be stronger?
Do you think the location of the slot could be a factor? (in the case of this bowl, the slot is in the end grain of the top piece and the padauk).

Thanks again for your time and insight.
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Ed Weber
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Re: Need your experience please - bowl cracked after 6 months
Reply #3 - Nov 30th, 2018 at 4:59pm
 
Alex Hixenbaugh wrote on Nov 30th, 2018 at 1:24pm:
I've been experimenting with using segmented feature rings in between these solid blocks of wood - do you think that would be stronger?


As long as it's not a single layer, at minimum two, otherwise instead of a crack, the segments will simply separate.
Alex Hixenbaugh wrote on Nov 30th, 2018 at 1:24pm:
Do you think the location of the slot could be a factor? (in the case of this bowl, the slot is in the end grain of the top piece and the padauk).


If it's going to open up or crack, it's going to do that at the weakest point. The weakest point is the end-grain where the fibers are the shortest.
You cut a slot, (intentional crack) which can relieve enough of the internal stresses that the wood will settle and won't open on it's own.
On the other hand, you cut a slot (relieved the internal stress) and now there is nothing restrain the wood from moving even further, so it did.
Most times in this situation you'll see the yarn slot be a bit uneven in one direction or another after the wood has settled a bit, in your case it simply didn't stop.

The number one rule of woodworking is that wood moves. Sometimes it doesn't matter what you do to try and stop it, if wood wants to move, it usually will.
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Ron Sardo
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Re: Need your experience please - bowl cracked after 6 months
Reply #4 - Nov 30th, 2018 at 5:52pm
 
Plywood always has an odd number of layers with alternating grain.
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Don Stephan
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Re: Need your experience please - bowl cracked after 6 months
Reply #5 - Dec 2nd, 2018 at 11:27am
 
The amount of offset at the crack immediately caught my eye.

Besides starting with similar moisture content in the three pieces of wood, different woods expand and contract radially and tangentially at different rates, so it is helpful to use woods with similar rates in both directions.
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Alex Hixenbaugh
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Re: Need your experience please - bowl cracked after 6 months
Reply #6 - Dec 3rd, 2018 at 11:23am
 
Thanks everyone.

What would be considered "similar" relative to shrinkage rates - within 10%? 20%? more?

The radial shrinkage rates of Bishopwood and Padauk are within ~20% but the tangential rate is up around 45%.
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